Method of servo-control in a braking system having electric brakes

ABSTRACT

The invention relates to a method of servo-control in a vehicle brake system including at least one electric brake having at least one actuator comprising a pusher facing friction elements and driven by an electric motor to apply a force selectively against the friction elements in response to a braking setpoint, the method making use of a plurality of relationships between various operating parameters of the actuator including a relationship between a pusher position and a corresponding force applied by the pusher to the friction elements. According to the invention, the method comprises the step of adjusting a particular relationship between position and force.

The invention relates to a method of servo-control in a braking systemhaving electric brakes.

BACKGROUND OF THE INVENTION

Modern aircraft have braking systems including electric brakes providedwith electromechanical actuators.

Each actuator comprises a pusher facing a stack of disks and moved underdrive from an electric motor to apply a braking force on the stack ofdisks in selective manner.

Such brakes are generally under force control based on a brakingsetpoint.

The invention applies more particularly to an actuator provided with asensor for sensing pusher position, but not including a force sensorcapable of measuring the force applied by the pusher against the stackof disks.

Under such circumstances, in order to be able to servo-control the forceapplied by such actuators, it is necessary to estimate the force beingapplied by a pusher as a function of parameters that can be measured,such as the position of the pusher, or indeed the power supply currentbeing drawn by the electric motor.

Alternatively, the braking setpoint can be converted into a positionsetpoint, whereupon position servo-control can be performed.

The servo-control that is implemented generally depends on parameters,relationships, and models that are estimated a priori. However, theconditions under which a brake operates can change during the lifetimeof the brake, thus making servo-control thereof less accurate.

OBJECT OF THE INVENTION

An object of the invention is to provide a method for countering apossible drop in servo-control performance during the lifetime of abrake.

BRIEF DESCRIPTION OF THE INVENTION

To achieve this object, the invention provides a method of servo-controlin a vehicle brake system including at least one electric brake havingat least one actuator comprising a pusher facing friction elements anddriven by an electric motor to apply a force selectively against thefriction elements in response to a braking setpoint, the method makinguse of a plurality of relationships between various operating parametersof the actuator including a relationship between a pusher position and acorresponding force applied by the pusher to the friction elements, andaccording to the invention, the method includes the step of adjustingsaid particular relationship between position and force.

It has been found that this relationship is very sensitive to frictionelement wear. Adjusting the relationship in accordance with theinvention makes it possible to compensate drift in the operatingconditions of the brake due to said friction element wear.

Preferably, the adjustment step comprises the following operations:

-   -   operating the brake under operating conditions in which the        force applied by the pusher against the friction elements        depends essentially on a power supply current flowing through        the electric motor;    -   in one or more positions of the pusher, measuring the power        supply current of the electric motor, and deducing a        corresponding force therefrom; and    -   from the position and force pairs determined in this way,        deducing a correction for the relationship between position and        force.

In a first particular implementation of the invention, said operatingconditions comprise moving the pusher at constant speed.

In a second particular implementation of the invention, the operatingconditions include one or more pauses in the position of the pusher.

In a third particular implementation of the invention, said operatingconditions comprise periodically displacing the pusher with smallamplitude about an operating point.

In which case, and preferably, the periodic displacement is implementedin superposition on a controlled displacement of the pusher in responseto the braking setpoint.

Advantageously, the adjustment step is implemented at least once perutilization cycle of the vehicle.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood in the light of the followingdescription given with reference to the figures of the accompanyingdrawings, in which:

FIG. 1 is a section view of an electric brake having electromechanicalactuators;

FIG. 2 is a block diagram of the servo-control used for controlling theactuators;

FIG. 3A is a graph showing how the position (bold line) and the powersupply current (fine line) vary as a function of time in a firstparticular implementation of the method of the invention;

FIG. 3B is a graph showing how the relationship between force and pusherposition is adjusted in the first implementation;

FIG. 4A is a graph showing variation in the position (bold line) and inthe power supply current (fine line) vary as a function of time in asecond particular implementation of the method of the invention;

FIG. 4B is a graph showing how the relationship between force and pusherposition is adjusted in the second implementation;

FIG. 5A is a graph showing variation in the position (bold line) and thepower supply current (fine line) vary as a function of time in a thirdparticular implementation of the method of the invention; and

FIG. 5B is a graph showing how a force/position pair is obtained in thethird implementation.

DETAILED DESCRIPTION OF THE INVENTION

The method of the invention is described in detail herein in applicationto an aircraft that has some number of braked wheels, of the kind shownin FIG. 1. Each of the braked wheels comprises a rim 5 suitable forreceiving a tire (not shown) and mounted to rotate on an axle 6 carriedby one of the undercarriages of the aircraft. The axle 6 has mountedthereon a ring 7 carrying actuators 8. A torsion tube 9 is secured tothe ring 7 and extends into the rim 5 and terminates with a backstop 10.The ring 7, and thus the torsion tube 9, are prevented from turningrelative to the axis 6 by keying means (not shown).

Between the rest 10 and the actuators 8 there extends a stack of disks11 made up of rotor disks that are constrained in rotation with the rim5, and stator disks that are constrained in rotation with the torsiontube 9.

Each of the actuators 8 comprises a body 12 in which a pusher 13 ismounted facing the stack of disks 11 to move linearly under drive froman electric motor contained inside the body 11 so as to apply a forceselectively to the stack of disks 11, which force, by inducing frictionforces between the rotors and the stators in the stack of disks,contributes to slowing down rotation of the rim 5, thereby braking theaircraft. Each of the actuators 8 includes a position sensor 14 formeasuring the linear displacements of the pusher 13.

The actuators 8 are associated with a control module 50 capable ofoperating in a controlled mode in which each pusher 13 is moved relativeto the stack of disks 11 by the associated electric motor in response toa braking setpoint which is generated in particular on the basis ofsignals coming from brake pedals 51 actuated by the pilot.

In such actuators, the torque imposed by the motor on the motor andgearbox unit for transforming the rotary motion of the motor into linearmovement in translation of the pusher is directly proportional to themagnitude of the current feeding the motor. This can be written asCem=K.i where Cem is the electromagnetic torque, K is a proportionalitycoefficient, and i is the power supply current drawn by the electricmotor.

Nevertheless, not all of the electromagnetic torque Cem is consumed inthe action exerted by the pusher 13 against the stack of disks. Somefraction of the electromagnetic torque Cem is consumed to overcomeinertial effects (acceleration or deceleration of the pusher and theassociated moving masses). Another fraction of the electromagnetictorque Cem is consumed to compensate for static friction and for viscousfriction (i.e. friction that depends on speed) opposing the displacementof the pusher 13. This can be written as follows:Cem=Ci+Cfs+Cfv+Cuwhere:

Ci is the inertial torque;

Cfs is the static friction torque;

Cfv is the viscous friction torque; and

Cu is the useful torque.

The useful torque gives rise to a force F such that Cu=a.η.F, where a isa transmission coefficient directly associated with the configuration ofthe motor and gearbox unit, and where η is the efficiency of saidtransmission.

The control module 50 is adapted to servo-control the actuators in themanner illustrated by FIG. 2.

The braking setpoint F is initially transformed into a position setpointx. For this purpose, use is made of a relationship R between theposition of the pusher 13 and the force exerted by the pusher 13 on thestack of disks 11.

This setpoint x forms the input to a position feedback loop. Thissetpoint is subtracted from the position x of the pusher 13 as measuredby the position sensor 14.

The resulting difference ε_(x) is processed by a first transfer functionG of the PID (proportional integral differential) type so as to betransformed into a current setpoint ī. This setpoint has subtractedtherefrom the current i as measured by the current sensor 15 which inthis case is integrated in the control module 50.

The resulting difference ε_(i) is then processed by a transfer functionH (a PID) and is then delivered to the electric motor of the actuator.

It has been found that the relationship R between position and the forceexerted by the pusher is particularly sensitive to disk wear. A specificobject of the invention is to adjust this relationship R to take accountof such wear.

For this purpose, and in the particular implementations described above,forces F_(p) corresponding to a plurality of positions x_(p) of thepusher are estimated, and the resulting estimated pairs (x_(p), F_(p))are used for adjusting the relationship R between position and force,e.g. by using a conventional regression method.

In a first particular implementation of the method of the invention asshown in FIGS. 3A and 3B, the pusher 13 is caused to advance at constantspeed.

Since its speed is kept constant, the effects of inertia are zero, suchthat the torque Ci is zero. Care is also taken to maintain the speed ata value that is low enough to ensure that the viscous friction force Cfvalways remains negligible.

In graph 3A, the bold curve representing the positions taken by thepusher 13 is then in the form of a straight line, with the time originbeing taken at the moment the pusher 13 comes into contact with thestack of disks 11.

During a first stage in which the pusher 13 is not in contact with thestack of disks, the useful torque Cu is zero, such that the entireelectromagnetic torque serves to overcome static friction. By measuringthe power supply current i₀ in this situation, a measurement is obtainedof the static friction:Cf=K.i ₀

The current i₀ is visible on the fine line curve for current. It is theconstant current taken by the motor before the pusher 13 comes intocontact with the stack of disks 11.

During a second stage in which the pusher 13 is in contact with thestack of disks 11, the useful torque is not zero and can be deduceddirectly from the measured current i:Cu=K.(i−i ₀)

In the invention, at a plurality of positions x_(p) for which the pusher13 is in contact with the stack of disks 11, the corresponding powersupply current i_(p) is measured while the pusher 13 continues to moveat constant speed. Each measured current is associated with a force bythe following relationship:F _(p) =K.(i _(p) −i ₀)/aη

In the graph of FIG. 3A, there can be seen for an instant t_(p), thecorresponding position measurement x_(p) (right-hand axis) and thecorresponding power supply current i_(p) (on the left-hand axis).

By repeating these measurements several times over, and by associatingeach measured current i_(p) with a force F_(p) using the aboverelationship, a series of pairs (x_(p), F_(p)) is obtained asrepresented by crosses on the graph of FIG. 3B. These pairs are used inthe invention to adjust the relationship R that is used in performingservo-control between position and force.

For example, if the servo-control makes use of a relationship R of theform x=αF+β, then the coefficients α and β are adjusted in conventionalmanner using conventional regression formulae.

This adjustment step is preferably implemented after the undercarriageshave been lowered and before the aircraft lands. Thus, the relationshipR between position and force is readjusted prior to each landing so asto take account of the state of wear of the disks.

In a second particular implementation of the method of the invention asshown in FIGS. 4A and 4B, the corresponding power supply current i_(p)is measured while the pusher 13 is held stationary in said position.Under such conditions, the inertial torque Ci and the viscous frictiontorque Cfv are zero.

To this end, and as can be seen in FIG. 4A, the pusher 13 is caused tomove with some number of pauses during which the pusher 13 remainsstationary. In the graph of FIG. 4A, there can be seen the positionx_(p) and the current i_(p) that correspond to an instant t_(p) takenduring one of these pauses.

By taking care to make use of positions in which the useful torque Cu ismuch greater than the static friction torque Cf, i.e. in which the powersupply current i_(p) is much greater than the current i₀, it is possibleto estimate a corresponding force F_(p) on the basis of the measuredpower supply current i_(p) by using the following relationship:F _(p) =K.i _(p) /a.η

By repeating these measurements several times over, and by associatingeach measured current i_(p) with a force F_(p) using the above-explainedrelationship, a series of pairs (x_(p), F_(p)) is obtained asillustrated by crosses on the graph of FIG. 4B. These pairs are used inaccordance with the invention to adjust the relationship R that is usedfor servo-controlling position and force.

In a third particular implementation of the method of the invention,while the pusher 13 is in a position in contact with the stack of disks11, a periodic displacement of small amplitude is applied thereto, asshown in FIG. 5A.

Under such circumstances, the inertial torque, the static frictiontorque Cfs, and the viscous friction torque Cfv have an average value ofzero, such that on average the useful torque Cu is directly equal to theelectromagnetic torque Cem.

The associated force is thus estimated by the following relationship:F _(p) =K.i _(p) /aη′where i_(p) in this case is an average of the power supply currentduring the periodic displacement of the pusher, and η′ is a weightedefficiency taking account of the fact that the efficiency of theactuator differs depending on whether the pusher 13 is moving in onedirection or in the opposite direction.

In order to establish this average, the power supply current i_(j) ismeasured at a variety of positions x_(j). The resulting pairs (x_(j),i_(j)) are represented by points in the graph of FIG. 5B. The mean ofthe positions x_(j) is then determined, which is retained as a measuredposition x_(p)=<x_(j)>, and the mean of the currents i_(j) is obtainedwhich is used as the associated power supply current i_(p)=<i_(j)>.

The force F_(p) is deduced from this current i_(p) in application of theabove-stated relationship.

By repeating this operation several times over, a series of pairs(x_(p), F_(p)) is obtained. These pairs are used in the invention toadjust the relationship R between position and force as used forservo-control purposes.

It should be observed that these measurements can be taken while theactuator 8 is being servo-controlled to track the braking setpoint F. Itsuffices to superpose small-amplitude periodic movements at a frequencyhigher than the frequencies characteristic of the braking system on thedisplacements of the pusher 13 in response to the braking setpoint F.

By adjusting the relationship between position and force of the pusherit is possible significantly to improve the performance of the electricbrake. By way of numerical example, on a brake for a commercial aircraftof the Airbus A320 or Boeing 737 type, the case is considered of abraking setpoint that is equal to the maximum force for which the brakeis deigned. At this setpoint, a fixed and non-adjustable relationship Rwould produce a displacement setpoint of the order of 2 millimetersregardless of the state of wear of the disks.

When implementing the step of adjusting the relationship R in accordancewith the invention, the displacement setpoint becomes about 1.6millimeters for a stack of new disks and about 2.3 millimeters for astack of worn disks, giving differences of the order of ±15% relative tothe setpoint obtained from a non-adjusted relationship.

At one extreme, this adjustment serves to avoid applying pointlessexcess pressure to the stack of disks, pointlessly fatiguing the brakeand prematurely wearing down the disks, and at the other extreme itavoids applying insufficient force, leading to poor braking performance.

The invention is not limited to the above description, and on thecontrary covers any variation coming within the ambit defined by theclaims.

In particular, although it is stated that the adjustment step isimplemented on each flight of the aircraft, i.e. on each utilizationcycle thereof, the adjustment step could be implemented in othercircumstances, for example in response to the friction elements crossinga wear threshold, or indeed periodically, once every ten flights or 100flights, or when replacing friction elements, or indeed when performingmaintenance in a workshop.

Although it is stated that a linear type relationship is used betweenpusher position and force, the invention is not limited to relationshipsof this type; the relationship could be non-linear.

Although the braking setpoint as described herein is a force setpoint,it is also possible to apply the invention when the setpoint isexpressed in terms of a percentage of a maximum force.

1. A method of servo-control in a vehicle brake system including atleast one electric brake having at least one actuator comprising apusher that faces friction elements and is driven by an electric motorto apply a force selectively against the friction elements in responseto a braking setpoint, the method making use of a plurality ofrelationships between various operating parameters of the actuatorincluding a relationship between a pusher position and a correspondingforce applied by the pusher to the friction elements, the methodcomprising: adjusting said particular relationship between said pusherposition and said corresponding force, wherein said adjustmentcomprises: operating the brake under operating conditions in which theforce applied by the pusher against the friction elements dependsessentially on a power supply current flowing through the electricmotor; in at least one position of the pusher, measuring the powersupply current of the electric motor and deducing a corresponding forcetherefrom; and from said at least one position and said correspondingforce, deducing a correction for the relationship between pusherposition and corresponding force.
 2. A method according to claim 1,wherein said operating conditions comprise moving the pusher at constantspeed.
 3. A method according to claim 1, wherein the operatingconditions include one or more pauses in the position of the pusher. 4.A method according to claim 1, wherein said operating conditionscomprise periodically displacing the pusher with small amplitude aboutan operating point.
 5. A method according to claim 4, wherein theperiodic displacement is superposed on a controlled displacement of thepusher in response to the braking force setpoint.
 6. A method accordingto claim 1, wherein the adjustment step is implemented at least once perutilization cycle of the vehicle.